1. Field of the Invention
This invention relates to a variable valve timing system for an internal combustion engine. More particularly, this invention relates to a torque pulse actuated, hydraulic variable valve timing system of the foregoing type with locking capabilities to lock the components of the system in a fixed condition of operation during intervals of low hydraulic pressure, such as during engine start-up.
2. Description of the Prior Art
U.S. Pat. No. 5,107,804 (Becker et al.), which is assigned to the assignee of this application, the disclosure of which is incorporated by reference herein, describes a vane-type, camshaft torque pulse actuated hydraulic camshaft or valve timing system for an internal combustion engine in which the hydraulic fluid that operates the camshaft phase shifting system is engine oil. Such a system has many operating advantages over other known types of valve or camshaft timing systems, for example, in the timeliness of response to changes in engine operating conditions. However, such systems tend to be noisy or otherwise unstable during periods of low engine oil pressure, which can often occur during engine start-up and can occasionally occur during other types of operating conditions. During these times it is important to be able to lock the otherwise relatively movable components of the system into fixed positions relative to one another, and it is to the provision of an improved solution of the system locking requirements of such a variable valve timing system that the present invention is directed and of suitable systems for controlling the operation of such a system.
U.S. Pat. No. 2,861,557 (Stolte) also describes an hydraulic variable camshaft timing system, albeit a system that is operated solely by engine oil pressure. This reference teaches that it is desirable to lock the otherwise variable components of the system in fixed positions relative to one another during low speed operation conditions, but only teaches a system in which only a single set of fixed positions can be achieved.